Presbycusis is characterized by deficits understanding speech in complex, acoustic backgrounds. Changes in monaural temporal acuity can explain some, but not all, of the speech processing problems. Preliminary studies show that the detection and localization of binaural transient auditory signals also deteriorate with age. Such deficits in binaural processing may add to the difficulties that older adults experience in understanding speech in everyday life. Therefore, our aim is threefold: a. First, we will determine in younger, middle-aged, and older adults the relationship between the detection of transient auditory signals and word recognition in fluctuating, acoustic backgrounds where the monoaural inputs to the binaural system are similar; b. Second, we will determine the relationship between the detection of temporal gaps and understanding of speech in fluctuating, backgrounds in younger, middle-aged, and older adults where the monoaural inputs to the binaural system contain interaural timing and intensity disparities and c. Third, if age-related changes in binaural temporal and speech processing are shown to occur, we will determine the decades during which the changes arise and whether these changes occur independently or are related. Rationale: In monaural and diotic listening conditions, there are no interaural disparities in the neural representation of the signal that serves as input to the binaural auditory system. In these conditions, age-related changes in gap detection thresholds and speech understanding reflect deficits in monaural, rather than binaural, processing. We have demonstrated that these monaural deficits in temporal and speech processing are present, even in older adults with normal hearing. However, in everyday listening conditions, temporal and intensity interaural disparities are present throughout the two monaural patterns that serve as the inputs for the binaural system. We hypothesize that age-related changes in gap detection thresholds and speech understanding, when determined in complex conditions that simulate everyday communication, will reflect not only those deficits already demonstrated in the monaural and diotic conditions but in addition, reflect age-related changes in binaural processing of great importance in everyday life. Also, we will audiologically characterize the representative samples of young, middle-aged, and old subjects with normal hearing and hearing loss who will undergo comparative experiments in psychoacoustics (this subproject), and speech perception (this subproject).